Subject: Returned mail: see transcript for details

From: Mail Delivery Subsystem <[email-masked]>
Date: Mon, 6 Oct 2025 09:23:39 +0300

The original message was received at Mon, 6 Oct 2025 09:23:26 +0300
from sink.edelkey.net [213.138.147.137]

   ----- The following addresses had permanent fatal errors -----
<[email-masked]>
    (reason: 550 5.4.1 Recipient address rejected: Access denied. For more information see https://aka.ms/EXOSmtpErrors [DU2PEPF00028D02.eurprd03.prod.outlook.com 2025-10-06T06:23:39.194Z 08DE036ABDBB2DCC])

   ----- Transcript of session follows -----
... while talking to halton-com.mail.protection.outlook.com.:
>>> DATA
<<< 550 5.4.1 Recipient address rejected: Access denied. For more information see https://aka.ms/EXOSmtpErrors [DU2PEPF00028D02.eurprd03.prod.outlook.com 2025-10-06T06:23:39.194Z 08DE036ABDBB2DCC]
550 5.1.1 <[email-masked]>... User unknown
<<< 503 5.5.2 Need rcpt command [DU2PEPF00028D02.eurprd03.prod.outlook.com 2025-10-06T06:23:39.194Z 08DE036ABDBB2DCC]